1. Technical Field
The present disclosure relates to a surge protection device and a field device including the surge protection device.
2. Description of the Related Art
In general, field devices include power supply terminals. Electrical power necessary for operation is supplied to the field device via a power supply cable connected to the power supply terminals. In two-wire transmitters, the power supply cable serves also as an output signal cable for a current that is variable within a range of 4 to 20 mA.
An instantaneous and high abnormal voltage called a surge voltage may be present in the power supply cable due to, for example, influence of lightning strikes or high-voltage facilities. In order to protect the field device from the surge voltage, a surge protection device (also called a lightning protector, arrester, surge absorber, transient protector, or the like) is used.
For example, the surge protection device is mounted on a circuit board of the field device, or is installed on the field device as a surge protection module. The surge protection device to be used as the surge protection module includes a pair of positive and negative power supply-side terminals to which the power supply cable is to be connected, and a pair of positive and negative device-side terminals to be connected to the power supply terminals of the field device to be protected. A surge protection element is connected between the positive and negative terminals.
As the surge protection element, a metal oxide varistor, an avalanche diode, a gas discharge tube, a surge protection thyristor, or the like is used. Further, a filtering function intended for noise reduction or the like may be incorporated into the surge protection device.
As a related document of this technical field, for example, JP-A-6-216433 is given.